Preheat oven to 350oF (180oC).
Cut chocolate into chunks and set aside.
In a bowl, whisk egg and sugar, until sugar dissolves and mixture is pale yellow.
Then, add cooled melted butter, milk and vanilla extract. Whisk until combined.
Sift flour and corn starch. Mix until just combined.
Add chocolate chunks and mix until just combined.
Transfer batter to piping bag. Skip this step if you are using an ice-cream scoop or spoon to drop dough on baking sheet.
Pipe dough 2 inch (5 cm) apart on a baking sheet lined with parchment paper.
Bake at 350oF (180oC). for 15 minutes, or until slightly golden around the edges.
Let cookies cool down on baking sheet for 10 minutes, then transfer to cooking rack to cool completely.